In color reproduction, it is often necessary to verify the color accuracy of separation negatives or positives used in the printing process. The use of overlay and surprint proofing systems for this application is well known in the literature. Even though surprint proofing systems have by far received the most acceptance for application where color assessment is critical, overlay proofing systems provide a more economical and more productive method for non-critical color prepress applications. Some examples of different types of overlay proofing films which are wet processed are described in U.S. Pat. Nos. 3,136,637; 4,299,906; 4,469,772; 4,482,625; 4,487,823; 4,748,102; 4,952,482; 5,001,037; and DE 37 20 724 A1; DE 38 13 322 A1; EP 338,786 A2; EP 404,507 patent applications. The technologies used to image these films are photosensitive systems well known in the art. Overlay color proofing films which are based on dry peel apart processes provide another method of generating multicolored proofs. Some examples of these can be found in U.S. Pat. Nos. 4,174,216; 4,334,006; 4,396,700; and EP 385,466 A2 patent application. Even though the dry process peel apart systems provide a convenient method of generating images without the use of wet developers, the image quality of these system are generally not as good as those generated by wet processes.
Other improvements to overlay systems, as described in U.S. Pat. Nos. 4,634,652 and 4,748,101, have included treatment of the polyester support for increased transparency and clarity. The intent of these permanent treatments are to reduce the internal reflections within the composite proof to provide a better match to the printed sheet. Particulate layers are positioned either on the opposite side of the carrier from the photosensitive color layer is coated or lie below the photosensitive color layer between it and the carrier.
The use of protective layers for preventing blocking or protecting the image have been described in U.S. Pat. Nos. 4,719,169; 4,902,594; and 4,999,266. In each of these patents, the protective layer is a permanent layer that is applied after the image is formed. The layer provides no protection for the film prior to image formation and subsequent lamination. The layer consists of a non-water soluble thermoplastic resin which may or may not contain particulate matter.
The use of particulate matter in adhesives for anti-blocking characteristics is well known. A specific example of using organic polymeric beads with a narrow molecular weight distribution in the adhesive layer of a surprint color proof is described in U.S. Pat. No. 4,885,225. The narrow distribution of beads allows one to incorporate a low concentration of beads into the adhesive layer without introducing optical interference or reduction in adhesive performance.
In an overlay construction, unlike a surprint type film, an adhesive layer is not the top layer in the construction. The exposed coated surface on an overlay film prior to development is either a photosensitive layer, which may or may not contain a colorant, or an oxygen barrier layer. An oxygen barrier layer is necessary when photopolymers or monomers are used which are quenched by triplet oxygen during the photoimaging process, typically acryloyl and methacryloyl polymerizable systems. The surface of these layer is typically very smooth, making it difficult to achieve uniform and intimate contact with separation negatives or positives during vacuum drawdown prior to the imaging process. The oxygen barrier layers are comprised of water soluble polymers which may block when subjected to moisture if the smooth top layer is allowed to come into contact with a second smooth surface. When photopolymers or monomers are used as the photosensitive element in the construction, the photosensitive layer and oxygen barrier layer are often soft and easily embossed during the photoimaging process. When non-uniform contact between the separation and photosensitive element occurs, a pattern results in the final image.